Recently, direct injection gasoline engines have been shown to improve engine performance and to reduce transient air-fuel disturbances that may be caused by fuel adhering to the intake manifold and ports of an engine. However, at higher engine speeds and higher engine loads, particulates may form in engine exhaust. Under some conditions, formation of the particulates may be related to the short amount of time between when fuel is injected to the cylinder and when combustion is initiated by a spark plug. Specifically, there may be only a short opportunity for the injected fuel to vaporize and form a homogenous mixture before combustion is initiated. If a homogenous air-fuel mixture is not formed in the cylinder before combustion is initiated, pockets of stratification may form, and soot may be produced by combusting rich areas within the cylinder air-fuel mixture. Particulate filters have been proposed as one way to reduce emissions of soot.
The inventors herein have developed a method for regenerating a particulate filter. In particular the inventors have developed a method for operating a spark-ignition engine having a three-way catalyst and a particulate filter downstream thereof, comprising: oscillating an exhaust air-fuel ratio entering the particulate filter to generate air-fuel ratio oscillations downstream of the particulate filter, while increasing exhaust temperature; when the downstream oscillations are sufficiently dissipated, enleaning the exhaust air-fuel ratio entering the particulate filter; and reducing the enleanment when an operating parameter is beyond a threshold amount.
By monitoring the air-fuel ratio downstream of the particulate filter for changes in exhaust air-fuel oscillations, suitable conditions for particulate filter regeneration may be identified. In particular, dissipation of the oscillations indicates oxidation of soot in the particulate filter. As such, enleanment of the exhaust air-fuel ratio may be accurately and timely performed to regenerate the particulate filter. For example, enleanment may be performed by introducing fresh air between the three-way catalyst and the particulate filter. Correspondingly, when an operating parameter is beyond a threshold amount indicating that the soot load has been oxidized, the enleanment may be reduced. For example, the introduction of fresh air may be reduced or stopped. In this way, changes in air-fuel downstream of the particulate filter may be leveraged to accurately enlean the exhaust entering the particulate filter to improve regeneration efficiency and reduce regeneration time.
As another example, the inventors have developed an engine system including: an engine configured to combust air and at least one of gasoline and alcohol; an exhaust system configured to receive exhaust from the engine, the exhaust system comprising: an emissions control device; a particulate filter positioned downstream from the emissions control device; an oxygen sensor positioned downstream from the particulate filter; and a temperature sensor configured to provide a temperature of the particulate filter; and a controller configured to, during regeneration of the particulate filter, increase a temperature of the particulate filter, and in response to a temperature of the particulate filter provided from the temperature sensor being greater than a temperature threshold and a time that a lambda of the oxygen sensor is biased rich is greater than a time threshold, introduce secondary air to a location downstream from the emissions control device and upstream from the particulate filter.
By monitoring changes in temperature of a particulate filter as well as changes of an oxygen sensor lambda downstream of the particulate filter, secondary air may be introduced between an upstream emissions control device and the particulate filter in an accurate manner to aid in regeneration of the particulate filter without disturbing the processing of engine exhaust gases in the upstream emissions control device. Accurate introduction of secondary air may increase the oxidation rate of soot in the particulate filter. In this way, particulate filter regeneration efficiency may be improved and regeneration time may be reduced. Moreover, the air-fuel control of the engine may be may be oscillated between lean and rich so that the emissions control device may process engine exhaust during particulate filter regeneration.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.